1. Field of the Invention
This invention relates generally to electron guns and is concerned more particularly with an electron gun supported within an electron tube envelope.
2. Discussion of the Prior Art
An electron tube of the cathode ray type may comprise a generally funnel-shaped envelope provided with a neck-end portion wherein an electron gun is axially disposed for directing an electron beam onto an anode target in the larger diameter end portion of the envelope. Terminating the neck-end portion of the envelope is a peripherally sealed glass disc or stem press through which extends a sealed array of electrical terminal pins. The terminal pins are connected electrically to electrodes of the gun including an electron-emitting cathode adjacent the stem press and a coaxial series of beam-forming electrodes.
The beam-forming electrodes may be provided with respective apertures which are axially aligned with one another for permitting passage of electrons in the beam and for focussing the beam onto a small circular spot area of the anode target. To aid in maintaining these apertures in the axial alignment, the beam-forming electrodes may be insulatingly attached to one another, as by axially extending dielectric rods, for example, to form a sub-assembly which is mounted on the stem press in alignment with the cathode.
In assembly, after the cathode and beam-forming electrodes are mounted on the terminal pins of the stem press, the beam-forming electrode most remote from the stem press is inserted into the neck-end portion and the stem press is urged axially to slide the gun into the neck-end portion. Consequently, the beam-forming electrode most remote from the stem press generally is provided with one or more radially projecting members which slidingly engage the inner surface of the neck-end portion to position the apertures in the electrodes of the sub-assembly substantially on the axial centerline of the tube. The beam-forming electrode nearest the stem press is fixedly attached in a relatively rigid manner to a plurality of terminal pins in the stem press and aids in sliding the gun axially within the neck-end portion as well as supporting the gun in axial spaced relationship with the stem press when installed in the neck-end portion.
However, it may be found that when the stem press is peripherally sealed to the encircling rim of the neck-end portion, it is very difficult to hold the stem press on the axial centerline of the tube. Due to the softening of the glass adjacent the periphery of the stem press during sealing, the stem press tends to move laterally or rotationally thereby exerting a corresponding radial pressure on the beam-forming electrode nearest the stem press. As a result, the apertures in the beam-forming electrodes adjacent the stem press are pulled out of axial alignment with the apertures in the beam-forming electrodes adjacent the opposing end of the sub-assembly. Thus, electrons passing through the apertures in the beam-forming electrodes adjacent the stem press may not pass through the apertures in beam-forming electrodes adjacent the opposing end of the gun. Consequently, the electron current of the beam may be unacceptably low; and the beam may be focussed onto an elliptical or a crescent-shaped area of the anode target.